1. Field of the Invention
The invention relates to an optical sensor device, a display apparatus, and a method for driving the optical sensor device, and in particular, to an optical sensor device suitable for a two-dimensional sensor such as a touch panel built into a display apparatus, and a method for driving the optical sensor device.
2. Description of the Related Art
An optical sensor made by a thin-film transistor (an optical sensor of a thin-film transistor type) detects a photoelectric current signal resulting from incidence of light on the optical sensor with a predetermined potential (normally a negative potential) applied to a gate of the thin-film transistor. The photoelectric current flows between a drain and a source of the thin-film transistor and is hereinafter referred to as a drain current. In recent years, various proposals have been made for a photodetection touch sensor constructed by incorporating such optical sensors into a display panel of a display apparatus.
Here, it is known that particularly a thin-film transistor (a-Si TFT) using amorphous silicon has its electrical characteristics changed by temporal changes or temperature changes. When such a temporal change or a temperature change occurs, a drain current output by the thin-film transistor before the change differs from that output by the thin-film transistor after the change, even with no change in the illuminance of incident light on the thin-film transistor. Such a change in the drain current from the thin-film transistor affects photodetection. Thus, if such a thin-film transistor is used as a touch sensor, a touch position may be erroneously detected or detection sensitivity may vary. Thus, the touch sensor may fail to operate stably.
Jpn. Pat Appln. KOKAI Publication No. 2009-87961 describes a configuration of an optical sensor that can control a change in drain current caused by a time-dependent degradation or a temperature-dependent change in the thin-film transistor. Jpn. Pat Appln. KOKAI Publication No. 2009-87961 describes the configuration corresponding to one optical sensor.
However, if the optical sensor is utilized as a two-dimensional sensor such as a touch panel, optical sensors need to be two-dimensionally arranged.
It is assumed that optical sensors are two-dimensionally arranged as described above. Then, if such a configuration as described in Jpn. Pat Appln. KOKAI Publication No. 2009-87961 is applied as an optical sensor and optical sensors are arranged so as to obtain high detection sensitivity, then an area required for interconnects connected to the optical sensors increases. Thus, the display quality of the display panel may be degraded. Furthermore, if the sensors and the interconnects connected to the sensors are arranged so as to avoid degrading the display quality, a sufficient number of optical sensors cannot be arranged. Hence, appropriate detection sensitivity cannot be achieved. Thus, it is difficult for the optical sensors to achieve appropriate detection sensitivity without degrading the display quality.